Despite its important role in the maintenance of healthy bone tissue, the osteoclast is still a poorly understood cell. The long term objective of this proposal is to increase our understanding of the regulation and development of osteoclasts. Our recent studies have shown that a well characterized hypercalcemia-inducing murine mammary tumor, CE mammary carcinoma, produces a new growth factor which stimulates bone marrow progenitors that give rise to cells with osteoclast characteristics. It is our hypothesis that this factor is the growth regulator of the osteoclast, "osteoclast-colony stimulating factor" (O-CSF), and is responsible for excessive bone resorption induced by this tumor. This hypothesis will be tested by further characterization of the responding cells in the bone marrow and definitive biochemical and functional studies of the molecule. The cells responding to this factor will be investigated by co-cultivation with bone pieces to demonstrate bone excavation. The osteoclast colony stimulating factor will be isolated and purified by biochemical protein separation procedures, followed by amino acid sequencing and cloning of cDNA; the recombinant molecule will be obtained by molecular cloning techniques. The physiological role of the growth factor for bone resorption and induction of hypercalcemia will be investigated by injecting the recombinant molecule into normal and osteopetrotic mice, by performing histochemical and morphometric analysis of bones and by analysis of serum calcium. Our approach to the studies of osteoclasts by clonal analysis of bone marrow progenitors will bring a new insight into the molecular regulation of osteoclasts. Certain tumors are abundant sources for growth factors and are known to influence functions of the host, such as severe hypercalcemia. Molecular characterization of the O-CSF in murine system will allow us to extend the studies of such a molecule in human in the future and will contribute to the understanding of various bone disorders, such as osteoporosis and osteopetrosis, as well as for humoral hypercalcemia of malignancy, a serious complication associated with certain cancer patients.